This invention relates to a process for cost-effectively producing commercial products from natural gas. More particularly, this invention relates to an improved process for manufacturing synthesis gas, an intermediate product useful for subsequent conversion to more easily stored and transported hydrocarbon products.
Natural gas generally refers to rarefied or gaseous hydrocarbons found in the earth. Non-combustible natural gases occurring in the earth, such as carbon dioxide,-helium and nitrogen are generally referred to by their proper chemical names. Often, however, non-combustible gases are found in combination with combustible gases and the mixture is referred to generally as xe2x80x9cnatural gasxe2x80x9d without any attempt to distinguish between combustible and non-combustible gases. See Pruitt, xe2x80x9cMineral Terms-Some Problems in Their Use and Definition,xe2x80x9d Rocky Mt. Minn. L. Rev. 1, 16 (1966).
Natural gas is often plentiful in regions where it is uneconomical to develop those reserves due to lack of a local market for the gas or the high cost of processing and transporting the gas to distant markets.
The technologies currently employed to move natural gas to remote markets generally include, but are not limited to, pipelines, the cryogenic manufacture of liquefied natural gas (LNG), and the production of Gas to Liquids (GTL) products. Each of these technologies feature benefits and incur penalties which can make one technology preferred over another for any given commercial environment.
Traditional GTL products include, but are not limited to, methanol, acetic acid, olefins, dimethyl ether, dimethoxy methane, polydimethoxy methane, urea, ammonia, fertilizer and Fischer Tropsch (FT) reaction products. The FT reaction produces mostly hydrocarbon products of varying carbon chain length, useful for producing lower boiling alkanes, alkenes, naphtha, distillates useful as jet and diesel fuel and furnace oil, and lubricating oil and wax base stocks.
GTL products, such as those produced from FT synthesis, including FT diesel fuels, have chemical and physical properties and environmental qualities that permit these products to most easily benefit from traditional energy infrastructure, including fuel storage and dispensing equipment. Additionally, fuel consuming equipment such as automotive/airplane engines as well as other utility, transportation and domestic power systems are currently more receptive or adaptable to the use of GTL products.
The broader implementation of GTL technology commercially, however, has been limited by the high capital cost and operational efficiency of the GTL manufacturing plants. Moreover, the largest single component of capital cost and the single largest contributor to operational efficiency (or inefficiency) has generally been the synthesis gas manufacturing units at the GTL manufacturing plant.
Therefore, there is a great need in industry for improved synthesis gas technology that is cost effective and operationally efficient.
For purposes of the present invention, methods for producing GTL products are categorized as either indirect synthesis gas routes or as direct routes. The indirect synthesis gas routes involve the production of synthesis gas comprising hydrogen and carbon monoxide as an intermediate product whereas the direct routes shall be construed as covering all others.
The most common commercial methods for producing synthesis gas are steam/methane reforming, auto-thermal reforming, gas heated reforming, partial oxidation, and combinations thereof.
Steam/methane reforming reacts steam and natural gas at high temperatures and moderate pressures over a reduced nickel-containing catalyst to produce synthesis gas where the reaction heat is applied externally to the process.
Autothermal reforming processes steam, natural gas and oxygen through a specialized burner where only a portion of the methane from the natural gas is combusted. Partial combustion of the natural gas provides the heat necessary to conduct the reforming reactions that will occur over a catalyst bed located in proximity to the burner.
Gas heated reforming consists of two reactors, a gas heated reformer reactor and an autothermal reformer reactor. Steam and natural gas are fed to the gas-heated reformer where a portion of the natural gas reacts, over catalyst, to form synthesis gas. This mixture of unreacted natural gas and synthesis gas is then fed to the autothermal reformer, along with oxygen, where the remaining natural gas is converted to synthesis gas. The hot synthesis gas stream exiting the autothermal reformer is then routed back to the gas reformer to provide the heat of reaction necessary for the gas-heated reformer.
Partial oxidation generally processes steam, natural gas and oxygen through a specialized burner where a substantial portion of the methane is combusted at high temperatures to produce synthesis gas. Contrary to autothermal reforming, no catalyst is present in the partial oxidation reactor.
Autothermal reforming, gas heated reforming, and partial oxidation all require an internal natural gas oxidation step where hydrocarbon is partially oxidized, generally in the presence of less than 60 mole % of the stoichiometric amount of oxygen necessary to fully oxidize natural gas to carbon dioxide and water. In each case, the hydrocarbon oxidation step results in a measurable portion of the hydrocarbon being converted into carbon dioxide. Carbon dioxide formation reduces the overall thermal efficiency of the GTL process resulting in lower product yields. The natural gas oxidation step, at high temperatures, can also lead to the formation of coke, soot, and coke and soot precursors that deactivate the synthesis gas catalyst, increase reactor pressure drop, and/or cause the synthesis gas reactors to plug.
Steam/methane reforming converts natural gas to synthesis gas in the presence of steam at elevated temperatures without the use of oxygen but with other means of external heat input. Since the steam/methane reforming reaction itself does not include internal oxidation or partial oxidation, substantially all of the energy required to facilitate the reaction must be supplied from an external source. These external sources generally include high reactor feed preheat temperatures supplemented by heat input supplied externally by the complete combustion of natural gas. In each case, the combustion products from the external heat supplied (water and carbon dioxide) are released to the atmosphere and are not converted to synthesis gas.
The steam/methane reforming reaction can also result in the formation of a synthesis gas having a higher hydrogen to carbon monoxide ratio than is desirable for certain GTL commercial uses. Such steam reforming processes often necessitate installation of equipment to remove excessive hydrogen prior to any downstream conversion steps or, after downstream conversion steps, result in unconverted hydrogen, carbon dioxide, light hydrocarbon and unrecovered synthesis gas being recycled within the process or consumed as fuel.
It has now been found that hydro-steam reforming utilizing steam recovered from the oxidation of hydrogen to water increases synthesis gas reforming process efficiency compared to steam/methane reforming processes requiring external energy sources that ultimately exhaust higher levels of carbon in the form of carbon dioxide to the atmosphere.
It has also been found that hydro-steam reforming utilizing the energy recovered from the oxidation of hydrogen to water substantially reduces or eliminates any external energy inputs that would otherwise be required to reach steam/methane reforming reaction temperatures or to sustain an endothermic steam/methane reforming process.
It has also been found that utilizing energy recovered from the oxidation of hydrogen substantially reduces coke and particulate accumulation in the reforming reaction zone compared to processes that oxidize substantial portions of natural gas hydrocarbon for reaction energy.
It has also been found that where hydro-steam reforming utilizing steam recovered from the oxidation of hydrogen results in a synthesis gas having a higher than stoichiometrically optimum molar ratio of hydrogen to carbon monoxide, that the excess hydrogen can be separated and synergistically oxidized to steam for further conversion to synthesis gas.
Therefore, the present invention is directed to a process for producing synthesis gas comprising the steps of reacting a hydrogen-containing stream with an oxygen-containing stream and producing an oxidized stream comprising water; contacting a feedstream comprising hydrocarbon or hydrocarbon comprising at least one atom of oxygen with the oxidized stream comprising water and forming a reforming feedstream; and passing the reforming feedstream into a reforming reaction zone at reforming reaction conditions and producing a synthesis gas product.
In another embodiment, the present invention is directed to a process for producing synthesis gas comprising the steps of oxidizing at least a portion of a hydrogen-containing stream with an oxygen-containing stream in an amount ranging from about 80 mole % to about 105 mole % of the stoichiometrically required amount of oxygen required to fully oxidize the hydrogen-containing stream to water, carbon dioxide or both and producing an oxidized stream comprising water at elevated temperatures; contacting a feedstock comprising hydrocarbon or hydrocarbon comprising at least one atom of oxygen with the oxidized stream comprising water and forming a reforming feedstream; and passing the reforming feedstream into a steam reforming reaction zone at steam reforming reaction conditions and producing a synthesis gas product.
In still another embodiment, the present invention is directed to a process for producing synthesis gas comprising the steps of oxidizing at least a portion of a hydrogen-containing stream with a stream comprising oxygen such that more than 60 mole % of the hydrogen-containing stream is oxidized to water, carbon dioxide or both in the reacting step and producing an oxidized stream comprising water at elevated temperature; contacting a hydrocarbon feedstream with the oxidized stream comprising water and forming a reforming feedstream; and passing the reforming feedstream into a steam reforming reaction zone at steam reforming reaction conditions and producing a synthesis gas product, wherein a substantial portion of the steam reforming heat of reaction is provided from the oxidizing step.
The hydro-steam reforming process in accordance with the present invention benefits from the oxidation of hydrogen to steam for supplying the energy necessary to initiate and sustain the endothermic steam methane reforming heat of reaction. The oxidation products of hydrogen (stream/water) are substantially friendlier to the environment than steam reforming processes which oxidize hydrocarbon to greenhouse gases such as carbon dioxide. Moreover, the oxidation products of the hydrogen-containing stream are consumed toward the production of synthesis gas compared to prior art steam reforming processes which generally exhaust hydrocarbon combustion products (carbon dioxide and water) to the atmosphere.
The hydro-steam reforming process in accordance with the present invention results in substantially improved operational reliability. The process of the present invention oxidizes clean burning hydrogen to provide the energy necessary to initiate and sustain the hydro-steam reforming reaction. Partial Oxidation, autothermal, and gas heated reforming processes oxidize hydrocarbon to sustain their synthesis gas conversion reactions resulting in the formation of coke, soot, and coke and soot precursors which contribute to catalyst deactivation, catalyst bed pressure drop/plugging, and reduced plant operability.
The hydro-steam reforming process in accordance with the present invention can be operated in a manner so as to manufacture its own hydrogen fuel requirements. The hydro-steam reforming process manufactures synthesis gas at a higher molar ratio of hydrogen to carbon monoxide than the stoichiometrically optimum ratio required for many of the known downstream GTL processes. In this manner, hydrogen separation steps may be incorporated to remove any excess hydrogen which may then be directed for the hydro-steam reforming reaction thereby producing a synthesis gas product having the optimum synthesis gas molar ratio of hydrogen to carbon monoxide for the particular downstream GTL process while minimizing or eliminating any need for externally supplied hydrogen.
The hydro-steam reforming process in accordance with the present invention requires substantially less steam/water addition than steam methane reforming and can be operated in a manner so as to be either a net consumer (or net producer) of water/steam. Since many downstream GTL operations, such as, but not limited to, Fischer Tropsch, methanol and DME manufacture generally operate as net exporters of waste water/steam, this water consumption capability synergistically and beneficially reduces raw water purification costs and the costs associated with waste water chemical addition.
The hydro-steam reforming process in accordance with the present invention can be designed in such a manner so as to eliminate the need for an external fired furnace. Through the optional use of supplemental steam and hydrogen addition, the hydro-steam process can generate sufficient hydrogen to entirely initiate and sustain the endothermic steam reforming reaction while providing sufficient hydro-steam reforming reactor preheat to eliminate the fired furnaces generally required to operate steam/methane reformers.